1. Field of the Invention
The present invention relates generally to the fields of virology, immunology and protein chemistry. More specifically, the present invention relates to a novel method of attenuating viruses.
2. Description of the Related Art
Vaccination is the best approach to reducing mortality and morbidity of humans caused by infectious diseases. Vaccines have made significant contributions to the eradication or control of several major infectious diseases in the world, such as smallpox, yellow fever, poliomyelitis and measles. In particular, live attenuated vaccines have been successful due to stimulation of different arms of the host immune response. These live vaccines are natural virus variants derived by passaging virus in abnormal hosts, e.g. yellow fever 17D virus in chicken tissue (Monath, 1990). However, this method is empirical, not cost-efficient, and takes a long time to develop a useful vaccine that can be administered to humans.
A virus can not infect a susceptible cell unless its viral attachment protein can bind to a molecule on the cell surface which serves as receptor for the particular virus. The expression of the receptor on specific cells or tissues in the whole host is a major determinant of route of virus entry into the host, the pattern of virus spread in the host and resulting pathogenesis (Marsh and Helenius, 1989; Lentz, 1989; Dimmock and Primrose, 1995). There are many factors determining host range and tissue tropism of viruses, including the nature, number and distribution of host cell receptor sites, which may be multivalent and consist of several receptor units (Paulson, 1985; Mims, 1986).
Japanese encephalitis (JE) is a disease caused by Japanese encephalitis virus, a mosquito-borne flavivirus which is epidemic throughout Asian countries (Huang, 1982) and the most common epidemic virus encephalitis in the world. The virus has a tissue tropism for the brain, in particular neurons. Japanese encephalitis virus belongs to genus flavivirus of the family Flaviviridae (Westaway et al., 1985). The genome of the virus consists of one single-stranded, positive sense RNA which is 10986 nucleotides in length and contains one long open reading frame that encodes three structural proteins at the 5' end (capsid (C), membrane (M) and envelope (E) proteins) and seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5) at the 3' end. The virus causes a neurological disease with a mortality rate of up to 50% and as many as 70% of survivors may develop neurological sequelae. There is no therapy to prevent the disease. Consequently, vaccines are used to control the disease (1). Attempts to generate live attenuated vaccines have proven difficult due to the neurotropism of the virus and the potential of reversion to virulence. Currently, little is known about the molecular basis of attenuation and virulence of Japanese encephalitis virus.
The prior art is deficient in the lack of effective means of attenuating viruses. The present invention fulfills this long-standing need and desire in the art.